1. Field of the Invention
The present invention relates to an improved method for forming a multi-layer, thin-film, metallic structures; more particularly to a structure that resists failure due to corrosion.
2. Description of the Prior Art
In the micro-electronics art, multi-layered, thin-film, metallic structures carry electrical signal and electrical power in a variety of applications, including but not limited to, attaching input/output pins to multi-layer ceramic modules in order to connect these pins to capture pads on the surface of the module. Corrosion is a problem with such multi-layer thin-film conductive structures particularly those structures that include copper. When such a multi-layer metal structure, even one insulated or separated by polyimides, is left exposed to the atmosphere, copper starts to diffuse. It will diffuse into a polyimide or other insulator where the copper reacts to form various oxides, and dendrites tend to grow in the insulator. These dendrites ultimately short out and corrosion occurs.
FIG. 1a illustrates one example of a prior art multi-layer structure of Cr/Cu/Ti/Au on a ceramic or a glass ceramic substrate. Here, copper from the conductive line and the pad react with a polyimide insulator and short out in the insulator by a dendritic corrosion mechanism. This takes place because gold, which is the top layer does not offer any protection to the layers underneath as it does not flow over the underlying metal deposits. Other multi-layer conductors exhibit similar corrosion characteristics. These include multi-layer structures of Cr--Cu--Ni--Au and Cr--Cu--Al--Au.
FIGS. 1b and 1c illustrate another prior art multi-layered thin-film structure of the type to which the teachings of this invention apply. Here, a capture pad 10 is formed by a suitable prior process on the surface of a multi-layer, ceramic module substrate 12. The capture pad 10 is itself a multi-layer metallic structure comprised of layers of chromium-copper-chromium.
After the capture pads are formed, a cushion layer of an insulating polymer, 12', typically a polyimide, is applied and cured over the entire area. Openings are then created in this polymer layer using a laser ablation process or a plasma or reactive ion etching process to expose contact bands on the capture pads. The purpose of the polymer layer is two fold. First, it is used to isolate the pin attachment stresses from the ceramic substrate, and second, it serves as a passivation coating to protect the capture pads.
At this juncture, the thin-film metallic bonding pad structure starts with a thin adhesion layer 11 of a suitable metal such as chromium. A relatively thick (e.g., approximately 6 .mu.m) stress reducing layer 13 of a soft metal such as copper is formed over the adhesion layer. To prevent the soft copper metal from reacting with a gold-tin eutectic alloy to be used subsequently to braze a pin to the structure, a reaction barrier layer 15 of titanium is then deposited over the stress reducing copper layer. The prior art process is completed by depositing a gold layer 17 over the reaction barrier layer. FIG. 1b illustrates this prior art structure prior to pin brazing. Here, it should be noted that the upper gold layer does not flow over the edge of the chromium, copper, titanium layers so that the edges of these layers are exposed. FIG. 1c indicates the structure of FIG. 1b generally by the reference number 18. In FIG. 1c a conductive pin 14 has been brazed to the structure 18 with a gold-tin braze. Here it should be noted that the brazing material 20 does not wet, and therefore, does not cover the edge of the Cr--Cu--Ti--Au structure 18.
Additional information with respect to prior art structures of the type shown in FIGS. 1b and 1c may be found in U.S. Pat. No. 4,835,593 entitled "Multilayer Thin Film Metallurgy for Pin Brazing", assigned to the assignee of this application and incorporated herein by reference.
While generally satisfactory, the prior art structures of the type described in connection with FIG. 1 are susceptible to corrosion and eventual failure during operation. One prior art approach to prevent corrosion is to coat the entire substrate surface with a polyimide coating. While generally satisfactory, the polyimide coating can randomly coat the contact surfaces, resulting in an insulated contact.